A system to operate and control a dual coil pressure intensifying fuel injector for an internal combustion engine is described in U.S. Pat. No. 5,954,030. This patent presents a system for operating a fuel injector that is capable of injecting fuel directly into a cylinder of an internal combustion engine. The system controls fuel flow by adjusting the position of a spool valve. The spool valve position is changed by flowing current to a charge coil or a discharge coil. When current is allowed to flow to the charge coil, the spool valve is attracted to the charge coil and fuel is allowed to enter an intensifier chamber. When current is allowed to flow to the discharge coil, the spool valve is attracted to the discharge coil and fuel is compressed in the intensifier chamber and released to the cylinder at a higher pressure. The charge and discharge coils position the spool valve so that the working fluid (i.e., pressurized oil), acts on the intensifier piston to compress the fuel in the intensifier chamber or to return the intensifier piston so that lower pressure fuel may enter the intensifier chamber. The pressurized oil acts on the intensifier piston and transmits force to a second piston that pressurizes incoming fuel. By transmitting force from the larger area intensifier piston to the smaller area second piston, the fuel pressure is multiplied. In addition, the patent provides for a method of holding the spool valve in place, using residual magnetism at the first coil, until current in the second coil reaches a level that can quickly move the spool valve when the magnetic force produced by the second coil exceeds the “latching” magnetic force at the first coil. The inventors claim that this method produces a snap action that improves the speed of injector operation.
The above system also has several disadvantages. Specifically, at lower ambient temperatures or when the viscosity of the working fluid increases, the meniscus forces and other forces in the oil that occupies the space between the spool valve and the valve body can increase to a level that may be difficult to overcome, even when the magnetic field from the coil is at a high level. Further, the fuel injector performance may degrade causing the engine air-fuel ratio to deviate from a desired engine air-fuel ratio. Consequently, engine performance and emissions may also degrade.